Forget tiny houses–meet the ADU.

We tend to think of housing as fitting into a few generic typologies: Single-family home. Apartment. Condo building. But in reality, the fabric of neighborhoods is made up of plenty of interstitial spaces that are neither here nor there–from the odd backyard shed to an alleyway addition to the hidden carriage house.

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Over the past few years, these alternative housing types–often known as Accessory Dwelling Units, or ADUs–have become better known, as demographics and socioeconomic forces reshape our communities. Cities including Los Angeles, Seattle, San Francisco, and Austin, among others, are all moving to ease restrictions on building ADUs.

ADUs are also becoming a mainstay of today’s architectural vernacular: Ben Waechter, a Portland-based architect, even won an AIA Award for an ADU in the backyard of a single-family home last year.

But with the increased attention has come debate–not only over how these buildings are regulated, but how they could potentially reshape cities themselves over coming decades. As that debate evolves, here are the basics.

Accessory Dwelling Unit is a technical name for an incredibly broad array of building types. It might be a shed in a backyard. It could be a carriage house behind an apartment building. Or an apartment above your garage. It could even be a tiny house in your backyard. An ADU is a housing unit on the same lot as a single-family dwelling. They’re also known as “in-law units,” “secondary units,” and a number of other nicknames. It wasn’t until the introduction of formal zoning regulations that they were broadly restricted or prohibited.

Some argue that ADUs are the calling card of the short-term rental market.

So why are you hearing so much about them lately? For one thing, the tiny house movement has moved various forms of alternative housing into the public eye. But ADUs aren’t just the domain of insufferable twenty-somethings who worship at the altar of minimalism. The AARP, for example, has been a prominent advocate for pro-ADU regulations, arguing that they allow older Americans to “age in place” and stay engaged in the fabric of society (they’re often called “granny flats” for a reason). As baby boomers age, and their children become homeowners and parents, the ADU is a natural way for grandparents to stay near their children without having to own their own homes.

Yet many experts are careful to note that ADUs aren’t a replacement for affordable housing–they may augment it. Martin John Brown, a researcher who blogs extensively about ADUs on AccessoryDwellings.org, points out that it’s difficult to compare ADUs to traditional housing types–because they’re built by homeowners, not real estate developers. He points to one study that found ADUs cost an average of 19% less to rent than non-ADUs in the Bay Area. In other cities, like Portland, they’ve just been average, though many rent for free or “ultra-low” costs. “Ultimately,” he wrote in a post this spring, “I feel the evidence shows ADUs have tremendous potential for creating reasonably priced housing.”

There are many reasons–just like there are many types of ADU–and every city and region is different, but many restrictions against these units emerged from zoning that tried to keep affluent, low-density neighborhoods that way. Most of the policies that outlaw ADUs around the country date back to the 1950s and ’60s, a time when suburbia was booming, and many Americans prized the notion of a single-family home in carefully orchestrated semi-isolation from the city center. In response, many areas introduced low-density zoning designed to preserve suburbia’s ratio of house-to-yard by restricting homeowners from further developing their land.

Portland is a major ADU hub, perhaps unsurprisingly, considering how the tiny house movement has boomed in the city. The city has waived the thousands of dollars in fees that a homeowner would normally pay to develop a new building on their land until 2018. “It’s a steep uphill battle to adopt good ADU codes since NIMBYs fear any sort of change to their single-family residential neighborhoods,” explains Kol Peterson, a Portland-based advocate for ADUs who collaborates with Brown on AccessoryDwellings.org. Peterson built his own backyard ADU, and in the years since, has become an outspoken proponent of the buildings. In Portland, he consults through his company Accessory Dwelling Strategies and leads day-long classes for people interested in building or converting their own ADUs (the next one is in September).

Peterson, who has a Master of Design Studies from Harvard’s Graduate School of Design, says he’s more interested in promoting the ideas behind ADUs than designing them himself. “At this stage, I feel that the impact of this advocacy and education work is more critical to my goal of seeing more ADUs built nationally than focusing my efforts on designing or building another ADU myself,” he says over email. “One of the best things someone can do to understand more about ADUs is to see them in person and talk to the homeowners, builders, and architects about their development process.”

The debate over ADUs is complex–like the land use laws and zoning policies that regulate them. Some critics worry the new construction will create overcrowding and burden city infrastructure.

Others argue that the ADU is the calling card of the short-term rental market. That’s been the hesitation of some in cities that range from Portsmouth, New Hampshire, to Asheville, North Carolina, and even in Palo Alto, as Peterson points out. “We want to know if we’re going to accomplish the goals of creating affordable housing or whether we’ll just create a lot of Airbnb opportunities,” a city commissioner, Asher Waldfogel, commented recently in Palo Alto Weekly.